In general, a metal can used as a storage container of beverage is mainly made of aluminum or iron.
FIG. 13 shows a metal can formed by a conventional technique and, as shown in FIG. 13, the conventional metal can has a basic body 2 of a bottle with a bottleneck and thus is called a necking can.
The metal can comprises a sealing lip 3 which is formed on the outer surface of the upper end of the bottleneck and connected to a cap to perform sealing, a beading part 4 which is formed at the bottom of the sealing lip 3 to determine whether the cap connected for the sealing is opened and to perform a pilfer-proof function, and a curling part 5 which is formed at the end of the bottleneck for the purpose of finishing.
In detail, the metal can of the type shown in FIG. 13 has the body 2 with the bottleneck by performing a drawing process and a necking process on a plate material. Moreover, the sealing lip 3 is formed by processing a fastening portion in the form of a screw on the outer surface of the upper end of the bottleneck, and the beading part 4 is formed by performing a beading process at the bottom of the sealing lip. Then, the curling part 5 is formed by performing the necking process again on the end of the bottleneck twice and then performing a curling process of bending and winding in the external direction.
Here, the process of forming the sealing lip 3 by processing the fastening portion in the form of a screw on the outer surface of the upper end of the metal can 1 for the connection with the cap is performed in the following manner. The sealing lip 3 is formed by inserting a screw-type inner die into the inside of the entrance of the bottleneck of the can, disposing an outer die close to the outside of the bottleneck of the can, and rotating the inner die and the outer die along the circumference of the can at the same time, followed by forming the beading part 4 at the bottom of the sealing lip 3.
However, according to the above-described conventional technique for processing the sealing lip of the metal can, since the processing of the sealing lip is performed by the rotational motion of the inner die and the outer die, the sealing lip can only be formed into a screw shape as shown in FIG. 13, and thus the can is sealed only with a corresponding screw cap, which imposes significant restrictions on the selection of the cap as well as on the function and design of a metal cap.
Moreover, since the processing of the sealing lip is performed by the rotational motion of the apparatus, the sealing lip is formed into only one shape. Thus, the shape of the sealing lip cannot be diversified, which severely restricts the use of the cap.
Furthermore, while the conventional apparatuses used in other processes in the production of the metal can perform a lifting and lowering motion, the above-described conventional apparatus for processing the sealing lip performs the rotational motion, which should thus be installed separately and require a separate power supply, which is also problematic.
This makes it difficult to provide a continuous process in a single system during the production of the metal can, which has been implicated as a factor that reduces the production efficiency and productivity.
In addition, according to the conventional technique, the process of forming the sealing lip is performed separately, and then the process of forming the beading part should be further performed. Thus, the production process of the metal can is complicated and takes a long time.
Additionally, when the pressure of the metal can produced by the conventional process of forming the sealing lip increases as gas is generated by microbial growth, heat, etc. in the metal can, the center of the cap, which is the most vulnerable part and connected for the sealing, is expanded, and the gas in the can is not discharged to the outside but exploded, which is very problematic.